Autism is a heritable neurodevelopmental disorder characterized by impairment in three domains including social interaction, communication and restricted repetitive and stereotyped patterns of behavior detectable before the age of three (American Psychiatric Association, 1994). Although the disorder is likely characterized by extensive genetic heterogeneity, certain traits such as language delay and types of repetitive behavior appear to segregate within families and may allow delineation of phenotypes with more homogeneous genetic determinants. In order to eliminate as much genetic heterogeneity as possible, we plan to investigate these endophenotypes, as well as autistic spectrum disorders, in the genetically isolated population of the Central Valley of Costa Rica. We will evaluate obsessive-compulsive symptoms, personality characteristics and language ability of first-degree family members to study genotype/phenotype correlations. Our goal is to perform a genomewide screen for autism susceptibility loci utilizing population genetic mapping methods in a sample of 175 trios; we also hope to fine-map loci discovered by other groups. In the process, we will examine the utility of recent multipoint methods of linkage disequilibrium analysis for the fine-mapping of disease loci using both microsatellites and single nucleotide polymorphisms.